Water soluble, crosslinkable polymer compositions, their preparation and use

ABSTRACT

The present invention relates to macromolecular compositions crosslinked by a novel bridge having the formula --NR x  --CH═N--CO--. This novel bridge is obtained by reacting a macro-molecule, preferably a polymer, having at least two amidocarbonyl groups with a compound having at least two formylamido groups in an aqueous medium under highly acidic conditions. The crosslinking bridge formed by the reaction of this invention is produced in accord with the following formula: ##STR1## Compositions containing macromolecules having at least two amidocarbonyl groups and a compound having at least two formylamido groups can act as viscosifying or gelling agents for aqueous acids where when added to aqueous acids, crosslinking occurs. Acids gelled in this manner have important use in the recovery of oil and natural gas from subterranean formations.

This application is a division of application Ser. No. 399,682 filedJuly 19, 1982, which in turn is a continuation of Ser. No. 216,672 filedDec. 15, 1980 and now abandoned.

The present invention relates to macromolecular compositions crosslinkedby a novel bridge having the formula --NR^(x) --CH═N--CO--. This novelbride is obtained by reacting a macro-molecule, preferably a polymer,having at least two amidocarbonyl groups with a compound having at leasttwo formylamido groups in an aqueous medium under highly acidicconditions. The crosslinking bridge formed by the reaction of thisinvention is produced in accord with the following formula: ##STR2##Compositions containing macromolecules having at least two amidocarbonylgroups and a compound having at least two formylamido groups can act asviscosifying or gelling agents for aqueous acids where when added toaqueous acids, crosslinking occurs. Acids gelled in this manner haveimportant use in the recovery of oil and natural gas from subterraneanformations.

The art of well stimulation can employ a technique calledfracture-acidizing to enhance the recovery of either oil or gas fromsubterranean carbonate formations. Carbonate formations includelimestone, dolomites, or other reservoir rocks which contain calcariousmaterial. Normally, fracture-acidizing involves the injection of anaqueous acid, which may or may not contain a proppant, into a wellboreat such a rate and pressure as to exceed the formation stresses therebycausing rock fatigue and inducing new fractures in the formation.Fractures are natural or induced cracks or channels in the formationmatrix. Stimulation by this technique is achieved by allowing the acidto etch the fracture face. Since the face is a heterogeneouscomposition, the acid reaction rates will vary on the exposed surface.After the exerted pressure has been relieved, fracture closure willoccur but the fracture face is no longer uniform and in most cases willnot perfectly align due to the action of the acid. If a proppant ispumped with the acid, the acid will again preferentially act on theformation face. However, since the formation is heterogeneous, someareas of the face will be nonreactive to the acid. After fractureclosure, the etched, non uniform localities of the face again will notalign while areas not etched will be "propped" open by that proppant.

In each case, a more conductive channel is provided to allow the oil orgas to flow to the wellbore after the injection pressure is relieved.When an aqueous acid is injected into a wellbore in a fracture-acidizingapplication, it is often advantageous to use a viscosifying or gellingagent in the fluid (c.f. U.S. Pat. Nos. 3,415,319; 3,434,971; 3,749,169;3,236,305 and 3,252,904). Viscous fluids possess several properties thatare favorable to fracture acidizing. For example, the fluid viscosity isproportionally related to the created fracture volume and fracturewidth, therefore higher fluid viscosities will generate larger fracturevolumes and fracture widths. In addition, viscous fluids decrease therate of the acid etching on the formation allowing the acid to penetratedeeply into the fracture and viscous fluids serve as efficient proppanttransporting media necessary to place proppant into the etched fracture.

Normally viscous fluids contain homo or copolymer compositions as theviscosifying agents. In particular, copolymer compositions containingamidocarbonyl pendent groups, such as polyacrylamide, are commonly usedas viscosifying agents in fracture acidizing fluids. Normally, lowpolymer concentrations, usually less than 1.0% by weight, are used. Toenhance the viscosity of the fluid, it is known from GermanOffenlegungsschrift No. 2,657,443 that copolymers of acrylamide arereadily crosslinked by the addition of aldehydes such as formaldehyde,acetaldehyde or glyoxal. The crosslinking reaction normally occurs atambient temperatures in alkaline solutions or between 40° and 80° C. inacidic media. However, heating the fluid to initiate crosslinking andthe time necessary to complete the crosslinking, sometimes in excess of45 minutes, make this fluid impractical in fracture acidizingapplications.

The present invention relates to new crosslinked polymer compositionswherein crosslinking is achieved by bridge members of the formula--NR^(x) --CH═N--CO, wherein R^(x) represents hydrogen or alkyl having 1to 4 carbon atoms or --CH₂ OH. The invention also relates to mixtures ofingredients, one comprising macromolecules having at least twoamidocarbonyl groups and another comprising a compound having at leasttwo formylamido groups, said mixtures being suitable for reaction in anaqueous medium under highly acidic conditions to crosslink with theformation of the --NR^(x) --CH═N--CO-bridge. The invention furtherrelates to methods of crosslinking such mixtures. The invention alsorelates to the use of the resulting crosslinked polymer compositions asviscosifiers for acids that do not have the disadvantages of the knownpolymers used for this purpose in fracture acidizing stimulation. Whilethe new crosslinked polymer compositions of this invention haveparticular utility as viscosifiers in fracture acidizing stimulation,they may also be used whenever it is desired to increase the viscosityof aqueous acids such as in the art of metal cleaning.

The compositions according to the present invention when employed inacid fracturing have the advantage that little or no precipitants areformed in the acid environment as is typical of previously known gellingagents. Precipitants, if formed, tend to plug the underground formationsand can severely limit the efficiency of the stimulation. In the presentinvention, if the crosslinked polymer composition degrades, theresulting polymer residues are soluble in the aqueous acid. This isbecause the crosslinking bridge decomposes leaving amino functionsattached to the polymer backbone. These amino functions impartsolubility to the polymer residues.

Crosslinkable mixtures of this invention preferrably comprise:

(a) a water-soluble homo- or copolymer having amidocarbonyl groups and

(b) a bis-acylamido-compound of the formula I ##STR3## wherein R¹ and R²represent hydrogen, alkyl having 1 to 4 carbon atoms or --CH₂ OH,

n represents a number from 1 to 3 and

m represents a number from 0 to 1

or a water-soluble homo- or copolymer having formylamido groups of theformula ##STR4## wherein R⁹ stands for hydrogen or alkyl having 1 to 4carbon atoms or CH₂ OH. The crosslinkable mixtures may contain one ormore of the components a together with one or more components b.

Component (a) of the crosslinkable mixtures of this invention andcomponent (b) are present in a ratio by weight of from 1:0.2 to 1:20,preferably 1:0.2 to 1:1.

In general, the homo- or copolymer of component (a) prior tocrosslinking may be any macromolecule, preferably a polymer, which issufficiently soluble in water and contains in the polymer an amount ofamidocarbonyl groups (at least two) sufficient to allow the formation ofthe novel crosslinking bridge, --NR^(x) --CH═N--CO--, in themacromolecule. Depending on which crosslinking agent is present in thecompositions R^(x) equals R¹, R² or R⁹. The solubility in water shouldbe not less than 10 g/l and the content of amidocarbonyl groups shouldbe above 10% preferably above 15%, calculated on the weight of themacromolecule.

Particularly suitable homo- or copolymers of the component (a) prior tocrosslinking comprise 50 to 100% by weight of entities of the formula II##STR5## wherein R⁴ stands for hydrogen or methyl and of 0 to 50% byweight of entities of the formula III ##STR6## wherein

R⁵ stands for hydrogen, alkyl having 1 to 3 carbon atoms, hydrogen andmethyl being preferred, and

Y stand for formylamido; N-substituted formylamido where theN-substitutent is methyl or hydroxy methyl, cyano, carboxyl or itsalkali, or ammonium salts, the alkoxycarbonyl group having 1 to 6,preferably 1 to 3 carbon atoms; the hydroxy-alkoxycarbonyl group having1 to 3 carbon atoms; the N-methylolamidocarbonyl group HOCH₂ NH--CO--,the methylol group of which may optionally be etherified with alkanolshaving 1 to 4 carbon atoms; the alkanoylamino group having 1 to 4 carbonatoms which may optionally be N-substituted with methylol or alkylhaving 1 to 4 carbon atoms; pyrrolidonyl-(1); phenyl; pyridinium; thesulfonic acid group; the sulfoalkylamidocarbonyl group having 1 to 4carbon atoms; the phosphonic acid group; it being possible for sulfonicand phosphonic acid groups to occur as alkali or ammonium salts; for aradical of the formula IV ##STR7## or for a radical of the formula V##STR8## and the quaternized compounds of (IV) and (V) quaternized withCH₃ --Cl or dimethyl sulfate, wherein R⁷ and R⁸ are the same ordifferent and stand for alkyl having 1 to 4, preferably 1 or 2; carbonatoms and p represents a number from 1 to 4.

Preferred homo- or copolymers of component (a) prior to crosslinkingcomprise 60 to 85% by weight of entities of the formula II and 15 to 40%by weight of entities of the formula III.

Furthermore, it is preferable to use as component (a) homo- orcopolymers in which

R⁵ signifies hydrogen or methyl and

Y signifies the carboxyl group; the sulfonic acid group,3-sulfo-2-methyl-propyl-(2)-amidocarbonyl of the formula ##STR9## analkanoylamino group having 1 to 4 carbon atoms which may optionally beN-substituted with methylol or alkyl having 1 to 4 carbon atoms;pyrrolidonyl-(1) or a radical of the formulas IV and V given and definedabove, where each acid group can also occur as Na-, K- or NH₄ -salt.

From among the group of compositions of this invention prior tocrosslinking containing a bis-acylamido compound of the formula I thoseare preferred which contain a compound of the formula I wherein

R¹ and R² represent hydrogen or --CH₂ OH, and

n represents the number 1.

It is possible to use as homo- or copolymer of component (b) prior tocrosslinking any polymer that is sufficiently soluble in water, i.e. notless than 10 g/l which in addition to formylamido groups also has anamount of amidocarbonyl groups sufficient to give stable crosslinkedpolymers of the novel chemical structure. Stated differently, it ispossible that component (a) and component (b) each contains the requiredamounts of amidocarbonyl and formylamido groups. Indeed, component (a)and component (b) may be the same. Self-crosslinkable copolymers of thisspecial class are described in a concurrently filed application entitled"Water-soluble Copolymers" in the names of Friedrich Engelhardt, KlausKuhlein, Ulrich Riegel, Sigmar P. Van Halasz, Jeffrey C. Dawson andAnthony R. Reed, the contents of which are hereby incorporated byreference and are to be considered an integral part of this disclosure.

Component (b) must contain a sufficient amount of formylamido groups toform the desired crosslink bridge. A sufficient amount of formylamidogroups should not be less than 1 gram mole of formylamido group (i.e.,71 grams of N-vinyl formamide) per 500 grams of the polymer.

Particularly suitable homo- or copolymers of the component (b) prior tocrosslinking comprise 50 to 100% by weight of entities of the formula VI##STR10## wherein R⁹ stands for hydrogen, methyl or hydroxymethyl,hydrogen and methyl being preferred, and of 0 to 50% by weight ofentities of the formula VII ##STR11## wherein

R¹¹ stands for hydrogen or methyl and

X stands for cyano; the carboxyl group or its alkali or ammonium salts;the alkoxycarbonyl group having 1 to 6, preferably 1 to 3 carbon atoms;the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; theamidocarbonyl group, the N-methylolamidocarbonyl group HOCH₂ NH--CO--,the methylol group of which may optionally be etherified with alkanolshaving 1 to 4 carbon atoms; an alkanoylamino group having 1 to 4 carbonatoms which may optionally be N-substituted with methylol or alkylhaving 1 to 4 carbon atoms; pyrrolidonyl-(1); phenyl; pyridinium, thesulfonic acid group, a sulfoalkylamidocarbonyl group having 1 to 4carbon atoms; the phosphonic acid group; it being possible for sulfonicand phosphonic acid groups to occur as alkali or ammonium salts; for aradical of the formula VIII ##STR12## wherein R¹² and R¹³ are the sameor different and stand for alkyl having 1 to 4, preferably 1 to 2 carbonatoms; for a radical of the formula IX ##STR13## wherein R¹² and R¹³have the meanings given above and p represents a number from 1 to 4; orfor a radical of the formula X ##STR14## and the quaternized compoundsof (IX) and (X) quaternized with CH₃ OCl or dimethyl sulfate, whereinR¹⁴ and R¹⁵ are the same or different and stand for alkyl having 1 to 4,preferably 1 or 2 carbon atoms and p has the meaning given above.

Preferred homo- or copolymers of the component (b) prior to crosslinkingcomprise 60 to 95% by weight of entities of the formula (VI) and 5 to40% by weight of entities of the formula (VII).

Furthermore, it is preferable to use as component (b) prior tocrosslinking homo- or copolymers in which X signifies the carboxylgroup, the sulfonic acid group,3-sulfo-2-methyl-propyl-(2)-amidocarbonyl of the formula ##STR15## analkanoylamino group having 1 to 4 carbon atoms which may optionally beN-substituted with methylol or alkyl having 1 to 4 carbon atoms,pyrrolidonyl-(1) or a radical of the formulas IX and X given and definedabove, where each acid group can also occur as Na-, K- or NH₄ -salt.

Preferably, copolymers are used as components (a) and (b) prior tocrosslinking having K-values of from 15 to 300 (cf. Fikentscher"Cellulosechemie" Vol 13, page 58 (1932)).

It stands to reason that the copolymers may contain several differentradicals X and Y respectively. As a rule, the radicals X and Y whenpresent in a single macromolecule have not more than 3, preferably 2,different meanings. Copolymers of this kind are prepared using severaldifferent comonomers carrying a radical X and Y respectively, as a rulenot more than 3, preferably 2.

The homo- or copolymer compositions of this invention prior tocrosslinking are readily soluble in water to give solutions of a mediumdegree of viscosity. Hence, these solutions can be handled easily. Thecompositions prior to crosslinking retain this property unless they aretreated with acids at a pH of 3 or below. Unlike the knownpolymer-aldehyde combinations, the compositions of this invention arestorable for an unlimited period of time. Acids are used to induce thenovel crosslinking reaction between the amidocarbonyl groups ofcomponent (a) and the formylamido groups of component (b) which resultsin a drastic rise in viscosity due to the formation of --NR^(x)--CH═N--CO-linkages forming a three dimensional novel polymer networkaccording to this invention. This is why these compositions are used toincrease the visosity of acids. Depending upon the amount of thecomposition added, the viscosity may be varied within a wide range. Thelower limit of this range is the viscosity of the pure acid free fromcopolymers. If, on the other hand, a sufficient amount of a compositionof this invention is added, gels of the novel cross linked polymercomposition may be obtained which do not flow spontaneously but keep anyshape given to them. Between these extremes any viscosity may beobtained by varying the composition contents of the acid.

In fracture-acidizing applications, the concentration of the acrylamidobromo or copolymer (component a) usually ranges from 0.24% to 0.72% byweight on total fracturing or treating fluid depending of the polymerproperties, well treating conditions or reservoir characteristics. Thecrosslinking agent (component b) will have a concentration range from0.05% to 1%, preferably 0.25% to 0.6%, based on total weight of thefracturing or treating fluid. The acrylamido homo or copolymers areintroduced into the aqueous phase as a fine solid powder, a hydrocarbondispersion containing 20% to 50% by weight of component a, or as an oilin water or water in oil emulsion normally containing 20% to 50% byweight of component a. The preferred system is an emulsion containingthe highest possible percentage of component a to produce a stableemulsion, normally 35% to 75% by weight. Suitable oils useful in suchwater in oil emulsions include normal or branched paraffinichydrocarbons having a boiling point of 150° C. to 250° C.

The crosslinking agents (component b) are normally introduced as aqueoussolutions containing up to 50% by weight of crosslinking agent, a finesolid powder, a hydrocarbon dispersion containing 20% to 50% by weightof crosslinking agent or water in oil or oil in water emulsionscontaining 20% to 50% by weight of crosslinking agent. The preferredstate would be as an aqueous solution containing 50% by weight activecrosslinking agent.

When the preferred concentration of acrylamido homo or copolymer, 0.24%to 0.72%, and preferred concentration of the crosslinking compositionsof this invention, 0.25% to 0.5%, are placed together in water, only amedium viscosity is obtained. A Fann 35 viscometer at a shear rate of511 sec⁻¹ will indicate a viscosity between 20 to 60 centipoise. Thecompositions will retain this viscosity until they are treated with aBronsted-Lowery acid. The acids are used to induce the crosslinkingreaction between the acrylamido homo or copolymer and the formylamidocompositions leading to the new crosslinked polymers of this inventionby the formation of linkages according to the following chemicalstructure --NR^(x) --CH═N--CO-- producing a drastic rise in viscosityusually exceeding 300 cps at a shear rate of 511 sec⁻¹ on a Fann 35viscometer.

The normal method of preparing the novel acid gel according to thisinvention is to add the preferred concentration of acrylamido homo orcopolymer to an acidic solution allowing the polymer to hydrate forusually 3 min. to 45 min. Generally, any acid or aqueous acid solutionmay be thickened according to the present invention. Infracture-acidizing applications, acid strengths normal range from 1/4%to 28% by weight of either hydrochloric, acetic or formic acid. Thepreferred strength is 3% to 15% depending on the well treatingconditions and reservoir characteristics. After 90% of the viscosityfrom the hydrating polymer has been obtained, the acidic solution istreated with the preferred concentration of crosslinking agent, 0.25% to0.5% by weight, a substantial increase in viscosity occurs according tothe formation of the new crosslinked polymer. It stands to reason thatany other way of combining the components of this invention with theacrylamido homo or copolymer and the acid likewise yields acid gels orhighly viscous acid preparations.

The viscosity of the acid thickened with the compositions of thisinvention is maintained at room temperature for long periods of time,e.g., exceeding 3 months. At elevated temperatures, e.g., above 50° C.,preferably above 80° C., the viscosity diminishes and the highlyviscous, gelled acids turn to liquids of low viscosity.

This change is due to a hydrolytical degradation of the novel threedimensional crosslinked polymer to linear polymer chains. The timenecessary for the change from the gel to the liquid state depends, to acertain extent, on the composition of the polymer molecule and hence,may be varied within certain limits, but not exceeding 12 hours, byselection of appropriate monomer compounds and monomer ratios.

Generally, any acid or aqueous acid solution may be thickened accordingto the present invention by forming the novel crosslinked threedimensional polymer. Acids being solid at normal temperatures, e.g.,aromatic sulfonic acids, have to be used as aqueous solutions. Preferredacids are those which are normally liquid and, with respect toeconomical use in the field of oil recovery, are strong and inexpensive.Hence in the first place, inorganic and strong lower organic acids aretaken into consideration.

Examples of acids which may be thickened according to this invention arehydrochloric acid, sulfuric acid, nitric acid, perchloric acid,phosphoric acid, formic acid, acetic acid, monochloroacetic acid,dichloroacetic acid and trichloroacetic acid.

Preferred acids for oil recovering pruposes are hydrochloric acid,formic acid and acetic acid.

The homo or copolymers of the component (a) prior to crosslinking areproduced by (co-)polymerizing 50 to 100%, preferably 60 to 85% by weightof acrylamide or methacrylamide and 0 to 50%, preferably 15 to 40% byweight of a comonomer of the formula IIIa ##STR16## wherein R⁵ and Yhave the meanings given above.

Monomers having the formula (IIIa) and suitable for copolymerizationwith acrylamide or methacrylamide include the following:

    __________________________________________________________________________    R.sup.5                                                                         H    CH.sub.3                                                                           H       CH.sub.3                                                                              H       H                                         Y CN   CN   COOH  ○.sup.1                                                                  COOH  ○.sup.1                                                                  COOCH.sub.3                                                                           NHCHO                                     R.sup.5                                                                         CH.sub.3 H        CH.sub.3 H       CH.sub.3                                 Y COOCH.sub.3                                                                            COOC.sub.2 H.sub.5                                                                     COOC.sub.2 H.sub.5                                                                     COOC.sub.4 H.sub.9                                                                    COOC.sub.4 H.sub.9                       R.sup.5                                                                         H      H        CH.sub.3 H        CH.sub.3                                  Y COOC.sub.6 H.sub.13                                                                  COOC.sub.2 H.sub.4 OH                                                                  COOC.sub.2 H.sub.4 OH                                                                  COOC.sub.3 H.sub.6 OH                                                                  COOC.sub.3 H.sub.6 OH                     R.sup.5                                                                         H         CH.sub.3   H          CH.sub.3                                    Y CONHCH.sub.2 OH                                                                         CONHCH.sub.2 OH                                                                          CONHCH.sub.2 OCH.sub.3                                                                   CONHCH.sub.2 OCH.sub.3                      R.sup.5                                                                         H              CH.sub.3       H                                             Y CONHCH.sub.2 OC.sub.2 H.sub.5                                                                CONHCH.sub.2 OC.sub.2 H.sub.5                                                                CONHCH.sub.2 OC.sub.4 H.sub.9                 R.sup.5                                                                         H          H          H          CH.sub.3                                   Y NHCOCH.sub.3                                                                             NHCOC.sub.2 H.sub.5                                                                      NHCOC.sub.3 H.sub.7                                                                       ##STR17##                                 R.sup.5                                                                         H        H        H        CH.sub.3                                                                              H                                           ##STR18##                                                                              ##STR19##                                                                              ##STR20##                                                                              ##STR21##                                                                             ##STR22##                               R.sup.5                                                                         H             H              H                                              Y                                                                                ##STR23##                                                                                   ##STR24##                                                                                    ##STR25##                                     R.sup.5                                                                         H         H       CH.sub.3                                                                              H        H                                        Y                                                                                ##STR26##                                                                               ##STR27##                                                                             ##STR28##                                                                             ##STR29##                                                                             SO.sub.3 H  ○.sup.1               R.sup.5                                                                         CH.sub.3      H              CH.sub.3                                       Y SO.sub.3 H  ○.sup.1                                                                  PO.sub.3 H.sub.2  ○.sup.1                                                             PO.sub.3 H.sub.2  ○.sup.1               R.sup.5                                                                         H                    H                                                      Y CONHCH.sub.2 SO.sub.3 H  ○.sup.1                                                            CONHCH.sub.2 CH.sub.2 CH.sub.2 SO.sub.3 H                                     ○.sup.1                                           R.sup.5                                                                       H                    H                                                      Y CONHC.sub.2 H.sub.4SO.sub.3 H  ○.sup.1                                                       ##STR30##                                               R.sup.5                                                                       H                    H                                                      Y CONHCH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2SO.sub.3 H  ○.sup.1                                   ##STR31##                                             R.sup.5        H                                                              Y                                                                                             ##STR32##                                                     R.sup.5                                                                         CH.sub.3            CH.sub.3                                                Y CONHC.sub.2 H.sub.4 SO.sub.3 H  ○.sup.1                                                     ##STR33##                                              R.sup.5                                                                         H                     CH.sub.3                                              Y                                                                                ##STR34##                                                                                           ##STR35##                                               R.sup.5                                                                      H                     CH.sub.3                                              Y                                                                                ##STR36##                                                                                           ##STR37##                                              R.sup.5                                                                       H                     H                                                     Y                                                                                ##STR38##                                                                                           ##STR39##                                              R.sup.5                                                                       H                     CH.sub.3                                              Y                                                                                ##STR40##                                                                                           ##STR41##                                              R.sup.5                                                                       H                     CH.sub.3                                              Y                                                                                ##STR42##                                                                                           ##STR43##                                              R.sup.5                                                                       H                     CH.sub.3                                              Y                                                                                ##STR44##                                                                                           ##STR45##                                              R.sup.5                                                                       H                     CH.sub.3                                              Y                                                                                ##STR46##                                                                                           ##STR47##                                              R.sup.5                                                                       H                     H                                                     Y                                                                                ##STR48##                                                                                           ##STR49##                                               R.sup.5                                                                      H                     H                                                     Y                                                                                ##STR50##                                                                                           ##STR51##                                              R.sup.5                                                                       CH.sub.3              H                                                     Y                                                                                ##STR52##                                                                                           ##STR53##                                            R.sup.5                                                                         CH.sub.3          H       H                                                 Y                                                                                ##STR54##                                                                                       ##STR55##                                                                             ##STR56##                                        R.sup.5                                                                         CH.sub.3 CH.sub.3         H                                                 Y                                                                                ##STR57##                                                                              ##STR58##                                                                                      ##STR59##                                        __________________________________________________________________________      ○.sup.1   Since Nvinyl amides are not stable under acidic             conditions, reaction mixtures containing acidic substances must be            neutralized prior to the polymerization such as with the above mentioned      basic comonomers.                                                        

The homo or copolymers of the component (b) prior to crosslinking areproduced by (co-)polymerizing 50 to 100%, preferably 60 to 95% by weightof a monomer having the formula VIa: ##STR60## wherein R⁹ has themeaning given above and 0 to 50%, preferably 5 to 40% by weight of acomonomer of the formula VIIa: ##STR61## wherein R¹¹ and X have themeanings given above.

Monomers having the formula (VIa) and suitable for the production ofhomopolymers of component b or copolymerization with monomers of theformula VIIa to form copolymers of component b are N-vinyl-formamide,N-vinyl-N-methyl-formamide and N-vinyl-N-hydroxymethyl-formamide.Monomers having the formula VIIa include acrylamide and the monomersillustrated above for formula IIIa.

Crosslinking of component a may also be achieved by employing compoundsof formula I as a crosslinking agent. Specific compounds of the formulaI include the following species:

    ______________________________________                                        R.sup.1      R.sup.2     n     m                                              ______________________________________                                        H            H           1     0                                              H            H           1     1                                              CH.sub.3     CH.sub.3    1     0                                              CH.sub.3     CH.sub.3    1     1                                              C.sub.2 H.sub.5                                                                            C.sub.2 H.sub.5                                                                           1     0                                              C.sub.2 H.sub.5                                                                            C.sub.2 H.sub.5                                                                           1     1                                              C.sub.3 H.sub.7                                                                            C.sub.3 H.sub.7                                                                           1     0                                              C.sub.4 H.sub.9                                                                            C.sub.4 H.sub.9                                                                           1     0                                              C.sub.4 H.sub.9                                                                            CH.sub.3    1     0                                              C.sub.3 H.sub.7                                                                            C.sub.2 H.sub.5                                                                           1     0                                              H            H           2     0                                              CH.sub.3     CH.sub.3    2     0                                              H            H           2     1                                              CH.sub.3     CH.sub.3    2     1                                              C.sub.2 H.sub.5                                                                            C.sub.2 H.sub.5                                                                           2     0                                              C.sub.3 H.sub.7                                                                            C.sub.3 H.sub.7                                                                           2     0                                              H            H           3     0                                              CH.sub.3     CH.sub.3    3     0                                              H            H           3     1                                              C.sub.2 H.sub.5                                                                            C.sub.2 H.sub.5                                                                           3     0                                              C.sub.3 H.sub.7                                                                            C.sub.3 H.sub.7                                                                           3     0                                              ______________________________________                                    

Preferred compounds of formula I are: ##STR62##(Methylene-bis-formamide) and ##STR63##(Bis-(N-formyl-aminomethyl)ether). The compounds of formula I can easilybe produced according to British Pat. No. 1,410,722.

If copolymers having several different radicals Y and X in components aand b respectively are desired, several different comonomers of theformula IIIa and VIIa respectively are used in the copolymerization, asa rule 3 or preferably 2.

The polymerization to produce components a or b may be performedaccording to any known polymerizing process. When one of the monomers isa vinyl-formamide monomer, a pH range from 6 to 12, preferably 7 to 9should be employed.

To adjust the pH value, alkaline reacting salts of alkali metals, e.g.,alkalicarbonates, alkalihydrogencarbonates, alkaliborates, di- ortrialkaliphosphates, alkalihydroxides, ammonia or organic amines of theformula NR₃ ¹⁶ are used, wherein R¹⁶ is hydrogen, alkyl having 1 to 4carbon atoms or hydroxyethyl whereby at least one of the radicals R¹⁶ isdifferent from hydrogen. Preferred bases for adjusting the pH value arethe alkali compounds mentioned above, especially sodium hydroxide,potassium hydroxide, sodium carbonate and hydrogen carbonate, potassiumcarbonate and hydrogen carbonate and sodium- and potassium borates.Another preferred base is NH₃.

The polymerization reaction can be initiated by energeticelectromagnetic or corpuscular radiation or by all substances which formradicals. Accordingly, possible polymerization initiators are organicper-compounds such as, for example, benzoyl peroxide, alkylhydroperoxides, such as, for example, butyl hydroperoxide, cumenehydroperoxide, p-menthane hydroperoxide, dialkyl peroxides, such asdi-tert.-butyl peroxide, or inorganic per-compounds such as, forexample, potassium, sodium or ammonium persulfate and hydrogen peroxide,and azo compounds such as, for example,azobisisobutyronitrile,2,2'azobis(2-amidinopropane)hydrochloride orazobisisobutyramide. It is advantageous to employ the organic orinorganic per-compounds in combination with reducing agents. Examples ofsuitable reducing agents are sodium pyrosulphite, sodium bisulphite orcondensation products of formaldehyde with sulphoxylates. Thepolymerization reaction can be carried out particularly advantageouslyusing Mannich adducts of sulphinic acids, aldehydes and amino compounds,such as are described in German Pat. No. 1,301,566.

It is known furthermore to add to the polymerization batches smallamounts of moderators, which harmonize the course of the reaction byflattening the reaction rate/time diagram, improve the reproducibilityof the reaction and hence lead to uniform products with extremely littlevariation in quality. Examples of suitable moderators of this type arenitrilo-trispropionylamide or hydrohalides of monoalkylamines,dialkylamines or trialkylamines, such as, for example, dibutylaminehydrochloride. In manufacturing the copolymers of the invention, suchcompounds can also be present with advantage.

Furthermore, so-called regulators can be added to the polymerizationbatches, that is to say compounds which influence the molecular weightof the polymers formed. Usable known regulators are, for example,alcohols such as methanol, ethanol, propanol, isopropanol, n-butanol,sec.-butanol and amyl alcohol, alkylmercaptans such as dodecylmercaptanand tert.-dodecylmercaptan, isooctyl thioglycolate, and some halogencompounds, such as carbon tetrachloride, chloroform and methylenechloride.

As usual, the polymerization is carried out in an atmosphere ofprotective gas, preferably nitrogen.

The reaction may be performed in solution, in emulsion or under theconditions of precipitative polymerization at a temperature of from 20°to 120° C., preferably from 40° to 100° C.

If water is used as a solvent for the reaction, the polymerization isrun in solution and a viscous, aqueous solution of the(co-)polymerizates is obtained. The product can be isolated either bydistilling off the water from the solution or by mixing the aqueoussolution with organic solvents miscible with water, whereby the(co-)polymer precipitates and can be separated from the liquid phase,e.g., by filtration.

It is preferred, however, to use the aqueous solution of the(co-)polymer obtained directly, optionally after having adjusted acertain desired concentration.

If (co-)polymerizing is performed in an organic solvent, as for instancein a lower alkanol, preferably, e.g., in tert. butanol, the reactionruns under the conditions of precipitative polymerization. In this case,the (co-)polymer formed precipitates from the starting solution as asolid compound during the course of the reaction. It can be easilyisolated in the usual manner, e.g., by filtration under suction anddrying the filter residue. Of course, it is also possible, and sometimespreferred, to distill off the organic solvent.

The following working examples demonstrate the use of compositionsaccording to the invention for the production of high-viscous acidcompositions, especially acid gels. Further illustrative workingexamples of the preparation of the polymers of component b, as well aspolymers of component a where Y is formylamido, can be found in theaforesaid application of Engelhardt et al entitled "Water SolubleCopolymers".

The abbreviations used in the examples and in the included tables havethe following meanings:

    ______________________________________                                        AM:    acrylamide                                                             VMA:   Nvinyl-Nmethylacetamide                                                AMPS:  2-acrylamido-2-methylpropane sulfonic acid                                    where the exponent                                                            1 signifies the ammonium salt,                                                2 signifies the salt with dimethyl-β-hydroxyl-                           ethylamine,                                                            AS:    acrylic acid                                                           MAS:   methacrylic acid                                                       NMA:   Nmethylol-acrylamide                                                   VSSNa: sodium salt of vinylsulfonic acid                                      NMVA:  Nmethylol-vinylacetamide                                               VA:    vinylacetamide                                                         VF:    vinylformamide                                                         NMVF:  Nmethylol-vinylformamide                                               VPA:   vinylphosphonic acid                                                   A:     ammoniumperoxidisulfate                                                B:     a combination of ammoniumperoxidisulfate + dibutyl-                           ammonium hydrochloride +                                                       ##STR64##                                                               C:   azo-isobutylronitrile                                                  VIP:   vinylpyrrolidone.                                                      ______________________________________                                    

EXAMPLE 1

(a) To 100 g of a 1% by weight aqueous solution of a copolymer of 80% byweight of acrylamide, 15% by weight of AMPS and 5% by weight ofvinylpyrrolidone (K=201) 5 g of N-methylene-bis-formamide are added. Theclear, slightly viscous solution thus obtained is stable and storablefor an unlimited period of time. Upon mixing this solution with 100 mlof concentrated hydrochloric acid the viscosity rises rapidly and within30 minutes a gel is formed. At ordinary temperatures of 20° to 25° C.the acid gel does practically not change its property over a period ofmore than 15 days. At 80° to 90° C., however, the gel degrades within 20minutes to give a slightly yellowish brown, clear liquid of lowviscosity. The copolymer used in this Examples can be produced asfollows:

(b) 600 ml of deionised water are first introduced into a 2 lpolymerisation vessel equipped with a stirrer, thermometer, gas inlettube, dropping funnel and heating bath, and the following monomers arethen introduced, while stirring the mixture and passing a light streamof nitrogen through it:

80 g of acrylamide,

15 g of AMPS and

5 g of vinylpyrrolidone.

Thereafter 0.5 g of azodiisobutyronitrile is added and the temperatureof the reaction mixture is brought to 50° C. by means of a heating bath.One ml of a 10% solution of benzoyl peroxide in acetone is then added tocause the polymerisation to commence, with a rise in temperature and inviscosity, a maximum temperature of 58° C. being reached.

After completion of the reaction, the mixture is stirred for a furtherhour at 80° C. The highly viscous polymer mass is now diluted to 1%strength by adding water, while stirring.

(c) Gels of similar properties are obtained if the copolymer used aboveis replaced by a copolymer of 55% by weight of acrylamide, 40% by weightof AMPS and 5% by weight of vinyl-methylacetamide (K=165) or with acopolymer of 60% by weight of acrylamide, 35% by weight of AMPS and 5%of vinyl-methyl-acetamide (K=179).

EXAMPLE 2

To a 3.5% by weight aqueous solution of a copolymer of 70% by weight ofacrylamide, 10% by weight of AMPS, 15% by weight of vinylformamide and5% by weight of vinylpyrrolidone (K-value=152) was admixed, withstirring, 1% by weight of N-methylene-bis-formamide. The clear solutionobtained was storable without change for an unlimited period of time.Upon adding thereto the same volume of concentrated hydrochloric acidthere was obtained, within a period of 20 minutes an acid gel which doesnot alter its state within a 10 day's period at 20° to 25° C.

At 80° to 90° C., however, the gel "fuses" within 20 minutes to give aclear liquid of low viscosity.

Acid gels of similar behavior may be obtained if the copolymer used inthis Example is replaced by the copolymers given in the following table:

    ______________________________________                                        AM       AMPS                ζ/rel.                                                                         k-value                                    ______________________________________                                        60       20      20 VIP                                                       55       20      25 VIP                                                       60       20      20 VIP       39.46                                                                              215                                        60       20      20 VIP      30.3  206                                        70       10      20 VIP      32.6  208                                        ______________________________________                                    

All amounts given in this table are parts by weight.

EXAMPLE 3

To 100 ml of a 1% by weight aqueous solution of a copolymer of 65% byweight of acrylamide and 35% by weight of sodium acrylate (K=203) 100 mlof concentrated hydrochloric acid are added. The strongly acidic mixtureobtained represents a slightly viscous liquid which can be storedunchanged for an unlimited period of time. Upon adding to it 0.5% byweight of methylene-bis-formamide, a gel stable in shape is formedwithin a period of 10 minutes. At 20° to 25° C. the acid gel remainsunchanged for at least 12 days, while at 80° to 90° C. a clear liquid orlow viscosity is formed within 30 minutes. A similar acid gel may beproduced if the methylene-bis-formamide used above is replaced by thesame amount of a compound having the formula ##STR65## The aqueouspolymer solution used can be obtained by adding, under vigorousstirring, 2.6 ml of a copolymer emulsion produced as described below,into 97.5 ml of water.

The copolymer emulsion is produced as follows:

1.85 g of sorbitan monostearate are dissolved in 30 g of a technicalisomeric hydrocarbon solvent having a boiling range of from 200° to 240°C.

The solution is introduced into a 2-liter reaction vessel fitted with amechanical stirrer, thermometer and gas inlet for nitrogen. A solutionof monomers is produced by dissolving 33.9 g of acrylamide and 18.2 g ofacrylic acid in 50 ml of deionized water and the pH-value of the mixtureis adjusted to 8.5 by adding a 20% aqueous solution of sodiumhydroxide.Then the aqueous monomeric solution is slowly added to the organicsorbitan monostearate solution while vigorously stirring and the air inthe reaction vessel is replaced by nitrogen.

0.07 g of 2,2'-Azobis-iso-butyronitrile, dissolved in acetone, are addedto the emulsion of the monomers and then the reaction vessel is gentlyheated to 60° C. while stirring. The polymerising reaction was finishedwithin 2.5 hours resulting in a stable 39% by weight strength emulsionof the copolymer having a K value of 203.

EXAMPLE 4

1% by weight aqueous solution of a copolymer of 90% acrylamide and 10%of dimethylaminoethyl-methacrylate (used for polymerising as theacetate) containing 5% by weight of N-methylene-bis-formamide was mixed,at a volume ratio of 1:1, with concentrated hydrochloric acid whichresults in an acid gel within 10 minutes. At 20° to 30° C., the gelobtained remains unchanged for more than 8 days but collapses to give aclear liquid of low viscosity at 80° to 90° C.

Replacement of the N-methylene-bis-formamide by the same amount of acompound having the formula ##STR66## leads to a similar result.

The aqueous polymer solution used can be obtained by adding, undervigorous stirring, 4.8 g of a copolymer emulsion produced as describedbelow, into 95.2 ml of water and subsequent addition of 5 g ofN-methylene-bis-formamide.

The copolymer emulsion is produced as follows:

185.0 g of a mixture of 84% strength saturated aliphatic hydrocarbonsand 16% strength naphthenic hydrocarbons (boiling point of the mixture:192° to 254° C.), 188.3 g of a chloroalkane having a chlorine content of66.5 and a density of 1.575 kg/m³ and 32 g of sorbitan monooleate aremixed in a vessel equipped with a stirrer, a thermometer and a nitrogeninlet and outlet.

After having well intermixed the constituents, a solution of 180 g ofacrylamide and 20 g of dimethylaminoethyl-methacrylate-acetate in 387.5parts of water is added and the aqueous phase is emulsified in theorganic phase. Nitrogen is allowed to pass through the mixture for 30minutes, which is then heated within 15 minutes to a temperature of 60°C. At this temperature a solution of 0.212 parts of2,2'-azo-bis-isobutyronitrile in a slight amount of acetone is added.After having heated the mixture to 60° C. for 3 hours, thepolymerization is complete.

A dispersion is obtained which does not clot, is safe againstsedimentation, has a viscosity of 710 m Pa sec. at 29.3 sec⁻¹ andcontains a polymer having a K value of 201. Polymer contents: 20.8%.

EXAMPLE 5

(a) 1.85 g of sorbitan monostearate are dissolved in 28 g of Isopar M, atechnical isomeric hydrocarbon solvent having a boiling range of from200° to 240° C., sold by Exxon Corporation. The solution is introducedinto a 2-liter reaction vessel fitted with a mechanical stirrer,thermometer and gas inlet for nitrogen. A solution of monomers isproduced by dissolving 33.9 g of acrylamide and 2.4 g of acrylic acid in40 ml of deionized water and the pH-value of the mixture is adjusted to8.5 by adding a 20% aqueous solution of sodiumhydroxide. Then theaqueous monomeric solution is slowly added to the organic sorbitanmonostearate solution while vigorously stirring and the air in thereaction vessel is replaced by nitrogen.

0.07 g of 2,2'-azobis-iso-butyronitrile, dissolved in acetone, are addedto the emulsion of the monomers and then the reaction vessel is gentlyheated to 60° C. while stirring. The polymerising reaction was finishedwithin 2.5 hours resulting in a stable emulsion of the copolymer.

(b) 67 g of Isopar M are introduced into a 1-liter polymerization flaskand heated to a temperature of 60° C., with weak stirring, 0.27 g ofsorbitan monolaurate, 1.3 g of sorbitan monostearate, 0.17 g of sorbitanmonooleate and 4.3 g of polyoxyethylene sorbitan monostearate beingsuccessively added and dissolved. With further weak stirring a stream ofnitrogen is passed into the solution and the temperature is adjusted to60° C.

94 ml of water are given into a separate vessel and 2.4 g of AMPS, 10.0g of VIMA, 10.0 g of vinyl pyrrolidone and 10.6 g of N-vinyl formamideare added and dissolved while stirring. This solution is adjusted to apH-value of 8 to 10 by the dropwise addition of 10% strength aqueoussolution of sodium hydroxide, 0.1 g of ammonium persulfate beingsubstantially added. This monomeric solution is then emulsified in theorganic phase via a dropping funnel in the reaction flask, with rapidstirring. The polymerization reaction starts after approx. 30 minutes,which is recognizable by a rise in the temperature. In the course of 15minutes the reaction temperature rises to 80°-90° C. The polymerizingreaction having faded out, the solution is heated for another two hoursat 80° C. A stable emulsion having a polymeric content of 30 percent byweight is obtained.

The two emulsions obtained under items (a) and (b) of this Example areintermixed and thoroughly homogenised by vigorously stirring the mixturefor 5 Minutes. The copolymer composition thus obtained has a copolymercontent of about 27% and represents a highly active, valuable thickeningagent for aqueous acids.

EXAMPLE 6

(a) 185.0 parts of a mixture of 84% strength saturated aliphatichydrocarbons and 16% strength naphthenic hydrocarbons (boiling point ofthe mixture: 192° to 254° C.), 188.3 parts of a chloroalkane having achlorine content of 66.5 and a density of 1.575 kg/m³ and 32 parts ofsorbitan mono-oleate are mixed in a vessel equipped with a stirrer, athermometer and a nitrogen inlet and outlet.

After having well intermixed the constituents, a solution of 212.5 partsof acrylamide in 387.5 parts of water is added and the aqueous phase isemulsified in the organic phase. Nitrogen is allowed to pass through themixture for 30 minutes, which is then heated within 15 minutes to atemperature of 60° C. At this temperature a solution of 0.212 parts of2,2'-azo-bis-isobutyronitrile in a slight amount of acetone is added.After having heated the mixture to 60° C. for 3 hours, thepolymerization is complete.

A dispersion is obtained which does not clot, is safe againstsedimentation, has a viscosity of 710 m Pa sec. at 29.3 sec⁻¹ andcontains a polymer having a K value of 201.

(b) 150 g of Exsol D, a deodorized kerosene boiling between 190° to 240°C., sold by Esso Chemie of Germany, are introduced into a 1-literpolymerization flask and heated to a temperature of 60° C., with weakstirring, 1.3 g of sorbitan monolaurate, 6.5 g of sorbitan monostearate,0.8 g of sorbitan monooleate and 22 g of polyoxyethlene sorbitanmonostearate being successively added and dissolved. With further weakstirring a stream of nitrogen is passed into the solution and thetemperature is adjusted to 60° C. 150 ml of water are given into aseparate vessel and 30 g of AMPS, 45 g of vinyl pyrrolidone and 55 g ofN-vinyl formamide are added and dissolved while stirring. This solutionis adjusted to a pH value of 8 to 10 by the dropwise addition of 10%strength aqueous solution of sodium hydroxide, 0.3 g of ammoniumpersulfate being subsequently added. This monomeric solution is thenemulsified in the organic phase via a dropping funnel in the reactionflask, with rapid stirring. The polymerization reaction starts afterapprox. 30 minutes, which is recognizable by a rise in the temperature.In the course of 15 minutes the reaction temperature rises to 80°-90° C.The polymerizing reaction having faded out, the solution is heated foranother two hours at 80° C. A stable emulsion having a polymeric contentof 30% by weight is obtained. The molecular weight of the polymer is95,000.

The two emulsions obtained under items (a) and (b) of this example areintermixed and thoroughly homogenised by vigorously stirring the mixturefor 5 minutes. The copolymer composition thus obtained has a copolymercontent of bout 27% and represents a highly active, valuable thickeningagent for aqueous acids.

EXAMPLE 7

A gas well in West Texas is selected to fracture-acidize. The well hasproduction interval from a depth of 9,650 to 9,740 feet. At thisinterval, the bottom hole static temperature is 170° F. while theformation permeability averaged 0.1 md. The well productivity prior totreatment is 4 MCF/day and 5 barrels of condensate by natural flow.

The fracture-acidizing fluid is prepared by blending 40,000 gallons of a15% hydrochloric acid solution containing 80 gallons, 0.2% by weight, ofa common corrosion inhibitor with 1,600 pounds (40 lb per 1000 gallons)of a fine powdered (100-120 mesh) copolymer. The copolymer contains 55%by weight acrylamide, 40% by weight of AMPS and 5% by weight ofN-methyl-N-vinyl acetamide (K=165). In addition, 80 gallons, 0.2% byweight, of a nonionic fluorosurfactant is also blended into the treatingfluid. After one hour of storage in two frac tanks, a low viscosityfluid (˜35 cps at 511 sec⁻¹ on a Fann 35) is obtained. Upon injection ofthe fluid into the well bore, the aqueous crosslinking solutioncontaining 50% by weight of active polymer from a composition of 80%N-vinyl formamide, 10% acrylic acid and 10% N-vinylpyrrolidone is addedat a rate of 4 gallons per 1000 gallons of treating fluid. Theinjections rate is 12 barrels per minute at a surface treating pressureof 6000 psi. After approximately 85 minutes, the well is shut-in for 6hours to allow the acid to react. After this time, the well headpressure is relieved and the well is placed back into a flowing status.The well productivity is tested and found to be significantly improved.

In its broadest aspects, highly viscous acid preparations or acid gelsof the present invention may contain from 70 to 99.8% by weight of oneor more of the aforementioned acids and 0.2 to 30% by weight of thecrosslinked polymer composition having characteristic --NR^(x)--CH═N--CO-- bridges. It is generally preferred that substantially all(at least 50% and preferably at least 70%) crosslink bridges have theaforesaid formula. Other known crosslink bridges may also appear in thefinal product depending upon the presence of other moieties incomponents (a) or (b) and/or the addition of known crosslinking agentsreactive with such moieties

As stated earlier, the amidocarbonyl groups of component (a) are carriedon macromolecules. This is because the macromolecule seems to stablizethe resulting bridge. Where the molecule to which the amidocarbonylgroups are attached is of insufficient size, no or only a small numberof desired bridges are stabilized, and the bridges appear to be degradedto formic acid, ammonium ion, amine, carboxylic acid and possibly otherdegradation products by hydrolytic action.

We claim:
 1. In a highly viscous aqueous acid composition comprising anaqueous solution containing 70 to 99.8% by weight of an acid and 0.2 to30% by weight of a crosslinked polymer composition, wherein theimprovement comprises said polymer containing crosslink bridges of theformula --NR^(x) --CH═N--CO-- wherein R^(x) represents hydrogen, alkylhaving 1 to 4 carbon atoms or --CH₂ OH.
 2. A highly viscous aqueous acidcomposition according to claim 1 wherein the crosslinked polymer isobtained by reacting in a highly acidic aqueous medium amidocarbonylgroups of component (A) with formylamido groups of component (B),component (A) comprising a water-soluble homo- or copolymer havingamidocarbonyl groups in an amount of more than 10%, calculated on theweight of the polymer and component (B) comprising a member selectedfrom the group consisting of(a) a bis-acylamido-compound of the formulaI ##STR67## wherein R¹ and R² represent hydrogen, alkyl having 1 to 4carbon atoms or --CH₂ OH,n represents a number from 1 to 3 and mrepresents a number from 0 to 1, and (b) a water-soluble homo- orcopolymer having formylamido groups of the formula ##STR68## wherein R⁹stands for hydrogen or alkyl having 1 to 4 carbon atoms or --CH₂ OH, theamount of formylamido groups being not less than 1 mol per 500 g of thepolymer.
 3. A highly viscous aqueous acid composition according to claim2 wherein component (A) comprises 50 to 100% by weight of entities ofthe formula II ##STR69## wherein R⁴ stands for hydrogen or methyl and of0 to 50% by weight of entities of the formula III ##STR70## wherein R⁵stands for hydrogen, alkyl having 1 to 3 carbon atoms andY stands forformylamido; N-substituted formylamido where the N-substituent is methylor hydroxymethyl; cyano; carboxyl; or the alkali or ammonium salts ofcarboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms; thehydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; theN-methylolamidocarbonyl group HOCH₂ NH--CO--; theN-methylolamidocarbonyl group wherein the methylol group is etherifiedwith alkanols having 1 to 4 carbon atoms; the mono- or dialkylaminogroup having altogether 1 to 8 carbon atoms in the alkyl radicals; thealkanoylamino group having 1 to 4 carbon atoms; the alkanoylamino grouphaving 1 to 4 carbon atoms which is N-substituted with methylol or alkylhaving 1 to 4 carbon atoms; pyrrolidonyl-(1); phenyl; pyridinium; thesulfonic acid group; the sulfoalkylamidocarbonyl group having 1 to 4carbon atoms; the phosphonic acid group; the alkali or ammonium salts ofsaid sulfonic acids and phosphonic acid groups; a radical of the formulaIV ##STR71## a radical of the formula V ##STR72## wherein R⁷ and R⁸ arethe same or different and stand for alkyl having 1 to 4 carbon atoms andp represents a number from 1 to 4; or quaternary salts of formulas IVand V.
 4. A highly viscous aqueous acid composition according to eitherclaim 2 or claim 3 wherein component (B) comprises 50 to 100% by weightof entities of the formula VI ##STR73## wherein R⁹ stands for hydrogen,methyl or hydroxy methyl and of 0 to 50% by weight of entities of theformula VII ##STR74## wherein R¹¹ stands for hydrogen or methyl andXstands for cyano; the carboxyl group; the alkali or ammonium salts ofcarboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms; thehydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; theN-methylolamidocarbonyl group HOCH₂ NH-CO-; the N-methylolamidocarbonylgroup wherein the methylol group is etherified with alkanols having 1 to4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms; thealkanoylamino group having 1 to 4 carbon atoms which is N-substitutedwith methylol or alkyl having 1 to 4 carbon atoms; pyrrolidonyl-(1);phenyl; pyridinium; the sulfonic acid group; a sulfoalkylamidocarbonylgroup having 1 to 4 carbon atoms; the phosphonic acid group; the alkalior ammonium salts of said sulfonic acids and phosphonic acid groups; aradical of the formula VIII ##STR75## wherein R¹² and R¹³ are the sameor different and stand for alkyl having 1 to 4 carbon atoms; a radicalof the formula IX ##STR76## wherein p represents a number from 1 to 4; aradical of the formula X ##STR77## wherein R¹⁴ and R¹⁵ are the same ordifferent and stand for alkyl having 1 to 4 carbon atoms and quaternarysalts of formula IX and X.
 5. A highly viscous aqueous acid compositionaccording to claim 2 wherein components (A) and (B) are present in aratio by weight of from 1:0.2 to 1:20.
 6. A highly viscous aqueous acidcomposition according to claim 2 wherein component (A) is awater-soluble homo- or copolymer having amidocarbonyl groups andcomponent (B) is a bis-acylamide-compound of the formula I ##STR78##wherein R¹ and R² are hydrogen, alkyl having 1 to 4 carbon atoms or CH₂OH.
 7. A highly viscous aqueous acid composition according to claim 6wherein component (B) is methylene-bis-formamide orbis-(N-formyl-aminomethyl)ether.
 8. A highly viscous aqueous acidcomposition according to claim 2 wherein component (A) is awater-soluble homo- or copolymer having amidocarbonyl groups andcomponent (B) is a water-soluble homo- or copolymer having formylamidegroups of the formula ##STR79## wherein R⁹ is hydrogen or alkyl having 1to 4 carbon atoms or --CH₂ OH.
 9. A highly viscous aqueous acidcomposition according to claim 2 wherein component (B) is a homopolymerof vinyl formamide.
 10. In the method of increasing the viscosity ofaqueous acidic solutions wherein a viscosifier is added to an aqueousacidic solution the improvement comprises using as the viscosifier acrosslinked polymer composition containing crosslink bridges of theformula --NR^(x) --CH═N--CO-- wherein R^(x) represents hydrogen, alkylhaving 1 to 4 carbon atoms or --CH₂ OH.
 11. The method according toclaim 10 wherein crosslink bridges of the formula --NR^(x) --CH═N--CO--are formed by the reaction in an aqueous acidic medium of at least twoformylamido groups with at least two amidocarbonyl groups.
 12. Themethod according to claim 10 wherein said crosslinked polymercomposition is obtained by reacting in a highly acidic aqueous mediumamidocarbonyl groups of component (A) with formylamido groups ofcomponent (B), component (A) comprising a water-soluble homo- orcopolymer having amido-carbonyl groups in an amount of more than 10%,calculated on the weight of the polymer and component (B) comprising amember selected from the group consisting of(a) a bis-acylamido-compoundof the formula I ##STR80## wherein R¹ and R² represent hydrogen, alkylhaving 1 to 4 carbon atoms or --CH₂ OH,n represents a number from 1 to 3and m represents a number from 0 to 1, and (b) a water-soluble homo- orcopolymer having formylamido groups of the formula ##STR81## wherein R⁹stands for hydrogen or alkyl having 1 to 4 carbon atoms or --CH₂ OH, theamount of formylamido groups being not less than 1 mol per 500 g of thepolymer.
 13. The method according to claim 12 wherein component (A)comprises 50 to 100% by weight of entities of the formula II ##STR82##wherein R⁴ stands for hydrogen or methyl and of 0 to 50% by weight ofentities of the formula III ##STR83## wherein R⁵ stands for hydrogen,alkyl having 1 to 3 carbon atoms andY stands for formylamido;N-substituted formylamido where the N-substituent is methyl orhydroxymethyl; cyano; carboxyl; or the alkali or ammonium salts ofcarboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms; thehydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; theN-methylolamidocarbonyl group HOCH₂ NH-CO-; the N-methylolamidocarbonylgroup wherein the methylol group is etherified with alkanols having 1 to4 carbon atoms; the mono- or dialkylamino group having altogether 1 to 8carbon atoms in the alkyl radicals; the alkanolyamino group having 1 to4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms whichis N-substituted with methylol or alkyl having 1 to 4 carbon atoms;pyrrolidonyl-(1); phenyl; pyridinium; the sulfonic acid group; thesulfoalkylamidocarbonyl group having 1 to 4 carbon atoms; the phosphonicacid group; the alkali or ammonium salts of said sulfonic acids andphosphonic acid groups; a radical of the formula IV ##STR84## a radicalof the formula V ##STR85## wherein R⁷ and R⁸ are the same or differentand stand for alkyl having 1 to 4 carbon atoms and p represents a numberfrom 1 to 4; or quaternary salts of formulas IV and V.
 14. The methodaccording to either claim 12 or 13 wherein component (B) comprises 50 to100% by weight of entities of the formula VI ##STR86## wherein R⁹ standsfor hydrogen, methyl or hydroxy methyl and of 0 to 50% by weight ofentities of the formula VII ##STR87## wherein R¹¹ stands for hydrogen ormethyl andX stands for cyano; the carboxyl group; the alkali or ammoniumsalts of carboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms;the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; theN-methylolamidocarbonyl group HOCH₂ NH--CO--; theN-methylolamidocarbonyl group wherein the methylol group is etherifiedwith alkanols having 1 to 4 carbon atoms; the alkanoylamino group having1 to 4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atomswhich is N-substituted with methylol or alkyl having 1 to 4 carbonatoms; pyrrolidonyl-(1); phenyl; pyridinium; the sulfonic acid group; asulfoalkylamidocarbonyl group having 1 to 4 carbon atoms; the phosphonicacid group; the alkali or ammonium salts of said sulfonic acids andphosphonic acid groups; a radical of the formula VIII ##STR88## whereinR¹² and R¹³ are the same or different and stand for alkyl having 1 to 4carbon atoms; a radical of the formula IX ##STR89## wherein p representsa number from 1 to 4; a radical of the formula X ##STR90## wherein R¹⁴and R¹⁵ are the same or different and stand for alkyl having 1 to 4carbon atoms and quaternary salts of formula IX and X.
 15. The methodaccording to claim 12 wherein components (A) and (B) are present in aratio by weight of from 1:0.2 to 1:20 .
 16. The method according toclaim 12 wherein component (A) is a water-soluble homo- or copolymerhaving amidocarbonyl groups and component (B) is abis-acylamide-compound of the formula I ##STR91## wherein R¹ and R² arehydrogen, alkyl having 1 to 4 carbon atoms or CH₂ OH.
 17. The methodaccording to claim 16 wherein component (B) is methylene-bis-formamideor bis-(N-formylaminomethyl)ether.
 18. The method according to claim 12wherein component (A) is a water-soluble homo- or copolymer havingamidocarbonyl groups and component (B) is a water-soluble homo- orcopolymer having formylamide groups of the formula ##STR92## wherein R⁹is hydrogen or alkyl having 1 to 4 carbon atoms or --CH₂ OH.
 19. Themethod according to claim 12 wherein component (B) is a homopolymer ofvinyl formamide.
 20. A highly viscous aqueous acid composition accordingto claim 1 wherein crosslink bridges of the formula --NR^(x)--CH═N--CO-- are formed by the reaction in an aqueous acidic medium of acompound having at least two formylamido groups with a polymer having atleast two amidocarbonyl groups.